The importance of being able to produce thinner dielectric layers is becoming increasingly recognized by the producers of multilayer capacitors (MLC's) due to end user requirements of reduced size and cost. These capacitors are typically manufactured by co-firing, i.e., sintering a ceramic dielectric formulation and a conductive electrode material in an oxidizing atmosphere at a temperature in the range of about 1000.degree. to 1400.degree. C.
Dielectric layers have traditionally been produced by preparing a suspension of ceramic powder in a liquid vehicle, usually containing a dispersant, and then adding an organic resin matrix which functions to bind the ceramic particles within the suspension. A variety of methods are known for applying the suspension and binder mixture (hereafter defined as slip) to a substrate to form very thin layers of the suspended solids. Methods such as wet coating, tape-casting (casting), or doctor-blading are readily known to those skilled in the art. The thin, dried layers generally termed as green layers, may then be coated with conductive electrodes and stacked together with similar layers to form a green body. The stack is then trimmed and co-fired to produce a structure consisting of alternating layers of sintered electrode and dielectric which is finally leaded with end terminations to form the finished capacitor. Suspensions used for dielectric compositions in the past have used both aqueous and organic liquids, but because of the environmental and safety concerns, the tendency has been to increase the use of aqueous suspensions for making the dielectric layers.
Another trend in the capacitor industry has been to make the dielectric layers thinner to obtain more capacitance per unit volume. Therefore, the thickness of dielectric layers have been reduced from 25 microns to 10 microns. It is now desirable to reduce the thickness to 5 microns or less. These thinner layers necessitate the use of extremely small solid ceramic particles in the suspension to produce the required high density and fine grain size in the final fired layer. When ceramic powders are reduced to such small particle sizes, i.e., less than 0.5 microns, they tend to have a significant soluble portion that dissolves in an aqueous suspension thus causing chemical reactions with the dispersants and binders in solution.
Smaller particles are also more difficult to handle making automated systems unduly complex and expensive.
Barium titanate, the base material of choice for capacitor formulations due to its dielectric characteristics, forms a soluble cation. Since the binder contains dispersing agents, any reaction of the soluble cation or its companion hydroxyl ion with the chemical dispersants in the binder can cause agglomerates of binder and "salting out" or precipitation of the metal cation--dispersant complex. These complexes or agglomerates often create voids in the ceramic body during the binder burnout phase prior to sintering and can result in either elevated levels of electrical leakage or electrical shorting paths. Void formation is particularly unforgiving in layers having a thickness of less than 10 microns.
Another problem that occurs when making suspensions with ceramic powder of less than 0.5 microns in diameter is that both the interfacial area between the solids and the liquid carrier and the number of particles in a given volume are greatly increased. This results in a high physical chemical interaction between the solid particles in the liquid phase, and diminished processability, especially at commercially acceptable solids loading levels. Hence it can be expected that the benefit of finer particle sizes can be countered by the necessity of going to lower solids loadings in the suspensions or slips. Manufacturing processes which expose the suspension to high shear conditions such as those encountered in pumping or tape casting, result in excessive gelling and in the worst case, dilatant-like conditions which are characterized by unworkable suspensions with shear thickening characteristics and high viscosities.
A variety of attempts have been made to prepare finely divided ceramic powders in aqueous suspensions and slips. For example, U.S. Pat. No. 3,496,008 discloses the ball milling of a ferroelectric material such as barium titanate in a 60% by weight solids loading level of milled material to water. The mixed suspension is rediluted to a desirable consistency for spray application.
In U.S. Pat. No. 3,551,197 a dielectric composition is prepared with between 40 to 90 weight percent of a ceramic powder in water. The ceramic powder is selected from a group including barium titanate, strontium titanate, calcium titanate, and lead titanate, and has a particle size of 0.5 to 3 micron. The suspended ceramic material is combined with a binder such as polymethylene glycol or diethylene glycol for example.
In U.S. Pat. No. 4,968,460, an aqueous emulsion of water soluble polymeric binder is combined with an aqueous suspension of ceramic material in a solids loading of at least 50 weight percent. The polymeric binder is used in a range of 0.5 to 35 weight percent and optionally with up to 5 weight percent of a selected dispersing agent. Tapes prepared from the slip composition had a thickness of between 30 microns and 2.540 mm. Particle sizes in the range of 0.5 to 12 micron are disclosed.
These references however, do not address the problems encountered in the preparation of aqueous suspensions or slips of ceramic powders having particles of less than 0.5 micron in diameter.
A suspension of ceramic powder having a diameter of 0.5 micron or less which remains suspended in an aqueous carrier fluid for extended periods of time in a substantially unagglomerated state and which maintains an apparent viscosity of less than 3000 centipoise (cps) without solidifying when determined from high shear rates of between 50 to 100/sec, would be a desirable improvement in the art of ceramic suspensions, slips, and the processes for producing them.
Yet another object of the present invention is to produce an aqueous suspension which has a ceramic powder loading of up to about 30% by volume and has an apparent viscosity of less than 3000 cps.
Yet another objective of this invention is to passivate the surface of the particle making up the ceramic powder with respect to soluble anions and cations, with a very thin layer of a relatively insoluble passivating agent, such as barium oxalate, then apply a dispersing agent to create a stable suspension. Surface passivation is necessary to prevent interactions of the barium, or other ions, which can cause cross-linking with the dispersing agent(s).
An additional objective of the invention is to enable the formation of stable slips at high solids loading of up to about 30% by volume of total solids (70 weight percent) or more of barium titanate, which have low enough viscosities to give good flow properties necessary to make thin green layers; which contain the necessary binder to form a cohesive film; and which have a uniform distribution of ceramic particles.
A further object of the present invention is to provide a slip which is non-dilatant in high shear applications such as pumping and spraying.